An immutable BiMap with reliable user-specified iteration order. Does
not permit null keys or values. An ImmutableBiMap and its inverse
have the same iteration ordering.

An instance of ImmutableBiMap contains its own data and will
never change. ImmutableBiMap is convenient for
public static final maps ("constant maps") and also lets you easily
make a "defensive copy" of a bimap provided to your class by a caller.

Note: Although this class is not final, it cannot be subclassed as
it has no public or protected constructors. Thus, instances of this class are
guaranteed to be immutable.

Returns an immutable bimap containing the same entries as map. If
map somehow contains entries with duplicate keys (for example, if
it is a SortedMap whose comparator is not consistent with
equals), the results of this method are undefined.

Despite the method name, this method attempts to avoid actually copying
the data when it is safe to do so. The exact circumstances under which a
copy will or will not be performed are undocumented and subject to change.

Serialized type for all ImmutableBiMap instances. It captures the logical
contents and they are reconstructed using public factory methods. This
ensures that the implementation types remain as implementation details.
Since the bimap is immutable, ImmutableBiMap doesn't require special logic
for keeping the bimap and its inverse in sync during serialization, the way
AbstractBiMap does.